Thoria-yttria emission mixture for discharge lamps

ABSTRACT

An improved emission material particularly useful as a coating on the metallic electrodes of metal halide lamps consists of a solid solution of yttrium oxide in the thorium oxide lattice. Enhanced maintenance including less shifting of color rendition with life is achieved. Preferred proportions are 5 to 20 mole percent of YO3 in ThO2 prefired into a single-phase solid solution.

States Patent 1191 Nov. 11, 1975 3.485.343 1.2/1969 3,530.327 9/1970Zollweg et 111. 313/346'R FOREIGN PATENTS OR APPLICATIONS 252.476 2/1970U.S.S.R 313/218 Primary Examiner-Saxfield Chatmon. Jr. Attorney, Agent.or Fir/11-Ernest W. Legree; Lawrence R. Kempton; Frank L. Neuhauser [57]ABSTRACT An improved emission material particularly useful as a coatingon the metallic electrodes of metal halide lamps consists of a solidsolution of yttrium oxide in the thorium oxide lattice. Enhancedmaintenance including less shifting of color rendition with life isachieved. Preferred proportions are 5 to 20 mole percent of Y O in Th0;prefired into a single-phase solid solution.

7 Claims. 4 Drawing Figures Jorgensen 313/218 X US. Patent, Nov. 11,1975 Sheet 1 012 THORIA-YTTRIA EMISSION MIXTUREFOR DISCHARGE LAMPSBACKGROUND OF THE INVENTION terial in the form of a refractorypowdermixture to lower the work function in order to start the are at lowervoltage. The. process of loweringthe starting voltages by applying lowerwork-function materials on the electrode surface is known as activationand an extensive literature exists on cathode activation theory andpractice. Concerning cathodes of interest for discharge lamps, theliterature includes the following references: E. S. Rittner, Philips.Res Reports, 8, 184 (1953);]. M. Lafferty, J. Appl. Phys., 22, 229(1951); 11 Speros, J. Electrochem. Soc., 106, 791 1959).

The search for better eleetrodeactivation materials has been acontinuing'one. Rare earth oxides including ThO and Y O have been usedseparately or together as emission mix'in high intensity discharge lampson account of their low work function, low vapor pressure and chemicalcompatibility withfthe lamp components and are materials. Some metalhalide lamps containing NaI, Tlland Inl as are materials used amechariical mixture of Th0,: and Y O with four rare earth oxides for theactivation material: Such emission mix containing six oxidesmechanically mixed was unsatisfactoryfin fact less satisfactory thanThO' alonei According to U.S. Pat. No. 3,530,327 Zollweg et al., rareearth oxides are most useful in-hi gh -i-nten'sity discharge lamps inwhich the halides of the same rare earths are used as are materials,that is, as the materialsthrough whose vapors the are discharge takesplace. In all of these previously known cases. a rareeai'th'oxide or amechanical mixture of one or more of theseoxides (including Th and Y Owas used asemission mix.

SUMMARY OF THE INVENTION The object of my invention is to provide a newemission material having improved characteristics and in particularbetter maintenance than what has been available up to now.

In accordance with my invention I provide a solid solution of yttriumoxide Y O in the thorium oxide ThO lattice, such material forming animproved electron emission material having enhanced maintenance andconstancy of color rendition when used for electrode activation in metalhalide lamps. The amount of Y O can range from a small amount effectiveto improve emission characteristics up to about 50 mole percent. I havefound the range from 5 to mole percent of Y O dissolved in ThO to beoptimum.

DESCRIPTION OF DRAWINGS FIG. 1 is a side view of a metal halide lamp inwhich the arc tube electrodes are activated with emission mix accordingto the invention.

FIG. 2 is a graph comparing the maintenance of tion.

lamps embodying theinvention with that of lamps using a mechanicalmixture of six oxides;

FIG. 4 isia section of a CIE chromaticity diagram comparing the lifecolor line of the lamp ofthe inven tion=withthat of a conventionalthorium oxide activated lamp.

DETAILED DESCRIPTION T heemission material of-the present inventionis asolid solution of Y O in-the Th0, prepared asfollows:

Weigh appropriate amounts of Y O and Th(NO .4H O and dissolve themixture in dilute nitric acid. Th(NO );.4H O is soluble in water at roomtemperature, whereas Y- O dissolves in warm dilute ni-' tric acid. Thesetwo materials 'may be dissolved separately and the solutions mixedtogether later; The mixed solution 'should be chilled in a refrigeratorfor several hours. The metal ions (Th,Y) nowcan be precipitated asoxalates by slowly adding a supersaturated solution of oxalic acid tothe chilled'nitrate solution kept in an ice bath. The nitrate solutionshould be con stantly stirred to keep oxalates fromforming lumps. Theprecipitate is now filtered, thoroughly washed and dried at C forseveral hours. The fine precipitate contained in a platinum'crucible isignited to oxides at 900C for several hours. The cooled oxide mixture isnew ground, blended and reheated at [500C for 3 4 hours; I

The material resulting from the foregoing preparation shows only onephase. namely ThO solid solution, u'iider X-ray diffraction analysis. Itis well known that the solubility of Y- O in the Th O lattice increaseswith the final sintering temperature (V. Subbarao et al., J.AmerJ'Ceramic Soc'., Vol. 48, No. 9, 1965). Thus at l500.and 1600C,aboutl2 and 2 0 mole percent of Y Qgre spectively can be dissolved inthe ThO lattice.

The maintenance of the ThO emission mix begins to improve as soon as theminutest'amountof Y O is incorporated into the lattice. Thus the amountof Y O can rangefrom a small amount effective to improve emissioncharacteristics up to about 50 mole percent which is the largest amountwhich can be incorporated into Th0 in practice. Depending upon thefineness of grinding, the thoroughness of mixing and the reaction timeand temperature, less than all of the Y O may go into solid solution inthe ThO In such case, the resultant material is a mixture of Y 0 andsolid solution of Y O in ThO It is the latter material that has the improved characteristics. I have found the range from 5 to 20 mole percentof Y O dissolved in ThO as a single phase solid solution to be optimum.

To activate an electrode, the sintered material is mixed with acetone,alcohol or some other organic binder into a slurry and milled forseveral hours to form a homogeneous suspension of appropriate particlesize. The metallic electrode is coated with the emission material bydipping it into the suspension up to a predetermined level; it is thenallowed to dry and any loose emission material is brushed off. Theamount of the emission material on the electrode surface variesaccording to the sizes of the lamp (wattage), arc cham- FIG. 3 is agraph comparing the maintenance of lattice which may be her, cathodeetc., and can be adjusted by controlling such factors as the density ofthe suspension and the particle size. The organic binder is burned offand the electrode degassed by thoroughly baking and sintering in thetemperature range from 1200 to 1900C in a vacuum environment prior toincorporation into the arc tube.

FIG. I shows an otherwise conventional metal halide lamp 1 of 400 wattsize in which my improved emission material may be used. It comprises anouter glass envelope 2 containing a quartz arc tube 3 having electrodes4, 5 sealed into opposite ends. The are tube contains an ionizablefilling comprising mercury, sodium iodide, thallium iodide, indiumiodide and an inert starting gas such as argon. As illustrated, theelectrodes may consist of two layers of tungsten wire turns wound overtungsten shanks 6, 7 which are connected through the usual molybdenumfoils to inleads 8, 9. The electrodes are coated with the ThO 0 emissionmix which also desirably fills the interstices between turns in the twolayers. The are tube inleads are connected to outer leads 10, ll sealedthrough press 12 of stem 13 of outer envelope 2. The outer leads in turnare connected externally to the contact surfaces of screw base 14attached to the neck end of the envelope.

Metal halide lamps using my emission material show better maintenanceand higher efficiency than prior art lamps. ln HO. 2, solid line 20represents the maintenance of 400 watt lamps such as illustrated in FIG.1 using the ThO .Y O emission material of the invention while brokenline 21 represents that of control lamps using pure ThO as emission mix.Many other high intensity discharge lamps are described in theliterature and the ThO .Y O material of the present invention can beused as a long-life source of thermionic elecline curve 24 representsthe color rendition with life of a lamp embodying the invention usingthe ThO .Y O emission mix, while broken line curve 25 represents that ofa conventional lamp using pure ThO- 1 The parenthetical numbers next tothe turning points give the lamp life in hours at those points. It willbe seen that the lamp according to the invention has less shift in colorfrom to 4000 hours, and less reversal in color from 4000 to 8000 hours.The change in the lamp according to the invention also tends to parallelthe black body locus which is a desirable characteristic.

What I claim as new and desire'to secure by Letters Patent of the UnitedStates is:

1. An electrode for an arc discharge device comprising a refractorymetal substrate and an electron emissive material carried thereoncomprising a solid solution of Y O in ThO wherein the proportion of Y Ois in the range from a small amount effective to improve emissioncharacteristics up to about 50 mole percent in ThO 2. An electrode as inclaim 1 wherein the proportion of Y O is in the range from 5 to 20 molepercent in ThO 3. A high pressure electric discharge lamp comprising alight-transmitting envelope having refractory metal electrodes sealedinto opposite ends and containing an ionizable medium including mercury,metal halide and an inert gas at low pressure, and an electron emissivematerial carried on said electrodes comprising a solid solution of Y Oin ThO wherein the proportion of Y Q is in the range from a small amounteffective to improve emission characteristics up to about 50 molepercent in ThO 4. A lamp as in claim 3 wherein the proportion of Y O isinthe range from 5 to 20 mole percent in ThO 5. A lamp as in claim 3wherein the refractory metal electrodes comprise turns of tungsten wirewound around a tungsten shank and the electron emissive material'iscoated on said turns and lodged in the interstices between turns.

6. A lamp as in claim 5 wherein the proportion of Y O is in the rangefrom 5 to 20 mole percent in ThO 7. A lamp as in claim 6 wherein theionizable filling includes sodium, thallium and indium iodides.

1. AN ELECTRODE FOR AN ARC DISCHARGE DEVICE COMPRISING A REFRACTORYMETAL SUBSTRATE AND AN ELECTRON EMISSIVE MATERIAL CARRIED THEREONCOMPRISING A SOLID SOLUTION OF Y2O3 IN TH2 WHEREIN THE PROPORTION OFY2O3 IS IN THE RANGE FROM A SMALL AMOUNT EFFECTIVE TO IMPROVE EMISSIONCHARACTERISTICS UP TO ABOUT 50 MOLE PERCENT IN THO2.
 2. An electrode asin claim 1 wherEin the proportion of Y2O3 is in the range from 5 to 20mole percent in ThO2.
 3. A high pressure electric discharge lampcomprising a light-transmitting envelope having refractory metalelectrodes sealed into opposite ends and containing an ionizable mediumincluding mercury, metal halide and an inert gas at low pressure, and anelectron emissive material carried on said electrodes comprising a solidsolution of Y2O3 in ThO2 wherein the proportion of Y2O3 is in the rangefrom a small amount effective to improve emission characteristics up toabout 50 mole percent in ThO2.
 4. A lamp as in claim 3 wherein theproportion of Y2O3 is in the range from 5 to 20 mole percent in ThO2. 5.A lamp as in claim 3 wherein the refractory metal electrodes compriseturns of tungsten wire wound around a tungsten shank and the electronemissive material is coated on said turns and lodged in the intersticesbetween turns.
 6. A lamp as in claim 5 wherein the proportion of Y2O3 isin the range from 5 to 20 mole percent in ThO2.
 7. A lamp as in claim 6wherein the ionizable filling includes sodium, thallium and indiumiodides.